The primary 5e sources (currently) for demonic lore are:
- Mordenkainen's Tome of Foes (MTF)
- Out of the Abyss (OoTA)
- Monster Manual (MM)
- Volo's Guide To Monsters (VGtM)
Creatures who have a lore link to Baphomet
From what I was able to find the creatures that follow Baphomet are:
- All the different types of demons (because the demon lords can dominate any demons they come across, and the Abyss has no particular structure, and thus no place where particular demons would normally inhabit)
- Cambions (See MTF's section on Demonic Cambions, under "Lords and Their Thralls")
- Molydeus (MTF stat block for the highest ranking demons below Demon Lords)
- other fiends including incubi, succubi, and night hags (MTF Blood War section)
- All manner of Beasts (inferred from his title as Prince of Beasts)
- Humanoid cultists (no specific race required)
- Orcs (with the addition of a Half-Demon/Half-Orcs called a Tanarukk)
- Minotaurs (Baphomet created them)
- Giants (from the MM and VGtM)
- other savage creatures (from the MM, which probably means Beasts)
- Molydeus (MTF: highest ranking demons below Demon Lords)
- Balor (MM: All Demon Lords likely have at least one Balor - see MM lore section)
- Goristro (MM: Given that they are described as resembling "a fiendish minotaur" and that they possess "preternatural cunning when navigating labyrinthine passages and shifting corridors" I'd say they are a pretty good thematic fit for Baphomet
- Mariliths (MM: often found as captains at the head of a demonic horde)
- Armanite (MTF: they are described as the heavy cavalry of the hordes, and they look like minotaurs crossed with centaurs...what more do you need for the Horned King and the Prince of Beasts??)
- Chasme (MM: described as interrogators/taskmasters)
- Hezrou (MM: Foot Soldiers of the demonic horde)
- Barlgura (MM: described as looking like a "hulking orangutan" and "moving apishly", which sounds like a perfect thematic fit for the "Prince of Beasts")
- Nalfeshnee (MM: described as being devastating in combat and working well in the thick of battle)
- Quasits (MM: spies/messengers, which is required for any confrontation)
- Tanarukk (MM: they are demonic half bred orcs which Baphomet is explicitly mentioned as gladly sharing the "secret of creating [them] to those who entreat him for power")
- Bulezau (MTF: their lore segment in MTF essentially mandates that some of these will be following Baphomet)
References for Lore
Out of The Abyss
I'm not going to spoil OoTA for myself (I want to play in that campaign at some stage), however that might be a place for the intrepid reader to do some further research on the Demon Lords and their minions.
Mordenkainen's Tome of Foes
MTF has extensive lore about the demons and the demon lords.
There is a large section of lore on the Blood War, which has this snippet of information relevant to your question:
[...] As creatures that don't favor either cause, because they care nothing for the philosophical concerns of law and chaos, other fiends including incubi, succubi, and night hags work for whichever side offers the best compensation. Demons use them as insurrectionists in the Nine Hells, inciting rebellion and defiance.
More important, demons that are slain and sent back to the Abyss return to their chaotic wanderings in that realm. A demon lord is thus hard pressed to keep a horde cohesive as it takes casualties. [...]
In the section on Demons the following piece of text (related to an incursion from the Abyss to a world on another plane) is relevant to your question:
[...] The lord's presence overwhelms the minds of other beings to keep them from resisting, and the lord's power enables it to command the other demons already present in the world. [...]
This suggests that you would expect to find all manner of demons following Baphomet as he would have the power to dominate any he came across and command them.
Further down in the "Evil Inchoate" section states:
[...] This self-centeredness applies even with regard to other demons. These fiends have no particular affinity for their own kind, which is the biggest reason why they seldom cooperate with one another unless they are forced to submit to a demon lord or other leader. [...]
Which again makes a point that you could find any demon in the service of a demon lord.
In the "Lords and their Thralls" section on Baphomet there are three pieces of relevant information:
Creatures he created:
[...] His fondness for labyrinths, instilled in the minotaurs he created, reflects this aspect of his personality. [...]
So Minotaurs are definitely in!
A selection of examples of his cultists:
[...] Cultists of Baphomet include nobles who use their vassals as playthings, assassins who practice their murderous art for the sheer love of hunting intelligent creatures, and paranoid humans who combine a hatred of outsiders with bloodthirstiness. [...]
Thus Humanoids of any variety are also in, as cultists of Baphomet.
There is a section on Demonic Cambions. Those Cambion's who serve Baphomet receive a special ability (which is detailed in the book):
[...] and one linked to Baphomet replaces it with Horned One's Call [...]
Among the new demons introduced in MTF are:
[...] In the armies of the demon lords, armanites perform the role of heavy cavalry, leading the charge and tearing into their enemies' flanks. [...]
Added to this is the fact that their picture looks like a cross between a centaur and a minotaur, I would say that they are a pretty good fit for Baphomet.
Their eagerness to kill and willingness to die make them common members of any demon lord's entourage.
This essentially mandates them being among Baphomet's followers.
MTF also introduces a stat block for extremely high-level servants of the Demon Lords:
The description for this new monster contains the following extracts:
The most ruthless and dangerous of demons - more feared than the dreaded balor - the molydeus speaks with the authority of the demon lord it serves as it enforces its master's will.
A demon lord has a direct link to its molydeus and uses the serpe nt head to communicate its wishes. A molydeus is, therefore, said to utter its master's will, commanding other demons to carry out orders and using violence to ensure they obey.
The weapon a molydeus wields reflects the nature of its master. Those that serve Baphomet carry a glaive;
Finally, it contains stat blocks for the demon lords themselves, along with accompanying lore. Under Baphomet's section his full title is given
[...] Baphomet, the Horned King and the Prince of Beasts [...]
This means it could be argued that any beast from the game could become a thrall of Baphomet...which significantly expands his potential for followers!
Additionally under Geryon's entry there is this snippet of Lore:
[...] Geryon has recently reclaimed his ancient fortress, Coldsteel, a sprawling complex that rises from the ice and snow at the center of Stygia. He roams the passages of this place, scattering the ice devils and minotaur slaves he took from Baphomet [...]
This gives us the information that Baphomet must have had ice devil slaves in his service at one stage.
There is some potential for debate on how this sentence should be parsed. There are three potential parsing options that I can see:
- ...scattering the (ice devils and minotaur) slaves he took from Baphomet
- ...scattering the (ice devils) and (minotaur slaves he took from Baphomet)
- ...scattering the ((ice devils) and (minotaur slaves)) he took from Baphomet
I’ve gone with option 1, suggesting both the ice devils and minotaurs were slaves of Baphomet and that they were taken from him
Option 2 would mean only the minotaurs were taken from Baphomet (and that they were slaves), but that the ice devils were always under Geryon’s purview
Option 3 would mean that Baphomet had both ice devils and minotaurs taken from him, but only the minotaurs were slaves
The MM has a 16 page section on Demons, which includes the following interesting lore snippets:
Demons respect power and power alone. A greater demon commands shrieking mobs of lesser demons because it can destroy any lesser demon that dares to refuse its commands.
By expending considerable magical power, demon lords can raise lesser demons into greater forms, though such promotions never stem from a demon's deeds or accomplishments. Rather, a demon lord might warp a manes into a quasit when it needs an invisible spy, or turn an army of dretches into hezrous when marching against a rival lord. Demon lords only rarely elevate demons to the highest ranks, fearful of inadvertently creating rivals to their own power.
It has a small lore snippet about Baphomet which offers some small additions to the MTF lore:
The demon lord Baphomet, also known as the Horned King and the Prince of Beasts, rules over minotaurs and other savage creatures. [...]
There are some relevant lore text segments about specific demons that are potentially relevant to this analysis:
Figures of ancient and terrible evil, balors rule as generals over demonic armies, yearning to seize power while destroying any creatures that oppose them. [...]
The lowly chasmes serve more powerful masters as interrogators or taskmasters. [...]
The goristro resembles a fiendish minotaur towering more than twenty feet tall. When controlled by a demon lord, goristros make formidable living siege engines and prized pets. Goristros possess preternatural cunning when navigating labyrinthine passages and shifting corridors [...]
The barlgura represents the savagery and brutality of the Abyss.
A barlgura looks like a hulking orangutan with a gruesome, drooping visage and tusks jutting from its jaw. Standing just under 8 feet tall, it has broad shoulders and weighs 650 pounds. It moves apishly along the ground, but it climbs with great speed and agility.
Hezrous serve as foot soldiers in the demonic hordes of the Abyss. [...]
Mariliths are often encountered as captains at the head of a demonic horde, where they embrace any opportunity to rush headlong into battle.
Nalfeshnees are devastating in combat, using their wings to soar above the front ranks and reach vulnerable adversaries that can be dispatched with little effort from the thick of battle, they telepathically bellow commands to lesser demons, even as they inspire a sense of dread that forces their foes to scatter and run. [...]
More powerful demons use quasits as spies and messengers when they aren't devouring them or pulling them apart to pass the time. [...]
The section on Giants has the following lore relating to Baphomet:
[...] Some giants abandon their own gods and fall prey to demon cults, paying homage to Baphomet or Kostchtchie. To worship them or any other non-giant deity is a great sin against the ordning, and almost certain to make a giant an outcast.
The section on Minotaurs has some more lore on their relation to Baphomet that we established in MTF.
Volo's Guide to Monsters
The section on Orcs from VGtM has a passage about Baphomet in relation to creating Tanarukks:
[...] A tanarukk is spawned when an ore tribe turns away from its gods and makes sacrifices to the demon lord Baphomet. [...]
and in the Tanarukk lore located with the stat block this link to Baphomet is reinforced:
[...] The demon lord Baphomet gladly shares the secret of creating tanarukks with those who entreat him for power. [...]
VGtM also confirms the MM snippet about some giants breaking away from their own gods and worshipping Baphomet.
Unless otherwise stated all emphasis has been added by me.
The implication of the 5e statement is, yes, that an evil god is trying to create something, and gets busy/bored/whatever and doesn’t finish, and somehow an atropal happens. As walen’s fine answer shows,¹ Lost City of Mezro suggests that the specific atropal found in Tomb of Annihilation might be “the Forsaken One,” a “godling born of a profane coupling [of] divine powers,” which doesn’t really seem “incomplete” to me, per se, nor does it really match the sense of “creation” in the implied sense. But then, the suggestion reads to me more like “hey this is a neat tie-in you could do in your game” than it seems like “here’s the canonical origin of that atropal.”
Anyway, prior to 5e, however, there was no implication that this was an abandoned creation. 3e’s Epic Level Handbook states only that they were “stillborn,” so again, not so much “creation” so much as “procreation.” And it need not have been the procreation of an evil god—any god who suffers the misfortune of a late miscarriage is at risk of a significant compounding trauma in the form of an atropal. (The only other mentions of the atropal in 3e that I can find are the 3.5e update pamphlet for Epic Level Handbook, and the atropal scion in Libris Mortis which mentions that the scion’s origin is from a bit of flesh from a destroyed atropal, but says nothing new about the atropal’s origins.)
In 4e, they avoid mentioning stillbirth, saying only that they are “unfinished godlings,” but “godlings” are generally created through reproduction so this to me sounds like a euphemism—there isn’t really any other way to have one that’s incomplete. This still doesn’t quite seem to match the story in Lost City of Mezro where the godling was successfully “born,” in my opinion.
It seems likely to me that the explanation here is just that they’re moving away from the specific, squicky, and potentially triggering, origin of atropals, towards something vague and vaguely compatible. That is, anyone familiar with and interested in using the 3e origin isn’t necessarily told not to by the 5e description (after all, as in 4e, “unfinished” could still be referring to the unsuccessful birth that would have “finished” it), though the mention of an “evil god” kind of throws a wrench in things a bit. But more importantly, anyone unfamiliar with, or uncomfortable with, the 3e origin doesn’t have it thrown in their face now.
Ultimately, though, that’s speculation. The atropal is not a “big,” popular monster with a lot written about it. It’s only really been discussed in any detail—and even then, not much of it—in three publications (3e Epic Level Handbook, 4e Monster Manual, 5e Tomb of Annihilation).² I cannot entirely rule out that it was discussed in some issue of Dragon or Dungeon magazine—this kind of thing would be in their wheelhouse, though again the atropal would be a pretty deep cut—but I’m pretty confident that you won’t find anything else officially discussing the monster.
walen’s answer also includes a transcription of a YouTube fan video that describes the relationship of the Forsaken One’s parents in more detail, and states that instead of being “born,” its mother “tore the unborn child from her womb.” This seems to be a contradiction of what’s said in Lost City of Mezro, and the video does not cite sources, so it isn’t clear whether or not this is original content or is based on some official source. It does notably match the 3e origins of an atropal better than what Lost City of Mezro says, however.
The 3.5e update for Epic Level Handbook only addresses the creature’s stats, in an highly “clinical” way. As mentioned, Libris Mortis mentions the atropal, but only in the context of producing the atropal scion and doesn’t discuss the atropal itself in any significant detail. And Lost Ruins of Mezro includes a bit about the specific atropal in Tomb of Annihilation, but it doesn’t discuss atropals in general much.